The goal of vegetable breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to insects or pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, growth rate and fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower of a different plant variety.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
One crop species which has been subject to such breeding programs and is of particular value is spinach. Spinach (Spinacia oleracea) is a flowering plant in the family Amaranthaceae native to central and southwestern Asia. Spinach is now grown in many temperate parts of the world, but is most productive in cool seasons and climates as heat will cause spinach to bolt. It is an annual plant (rarely biennial) having flowers that mature into a small hard dry lumpy fruit cluster about 5-10 mm across containing several seeds.
Spinach has two stages in its life cycle including the vegetative, rosette stage in which the plant is marketable (about 35-40 days) and the bolting, seed stalk stage in which the plant is no longer marketable. Spinach can grow in a range of soils as long as they are moist and fertile, and particularly sandy loams that are high in organic matter. However, the plants typically do poorly in acid soil, which should be at least a pH of 6.0, having an optimum pH of 6.2-6.9.
Spinach performs poorly against weeds. For weed control, usually 2-4 cultivations are conducted in spinach fields, but fields should be shallow as to not harm any of the roots. Spinach also has a shallow root system, so spinach grows best in uniformly moist conditions. Fields are irrigated by either flooding, furrow, or overhead sprinklers, furrow or flooding irrigation is preferred because overhead irrigation can reduce yields by increasing the risk and levels of disease that thrive in moist, humid conditions.
Spinach is traditionally classified by sowing time (spring, summer, and winter spinach) and harvesting method. Spinach is considered to be dioecious, although there is a continuous range of monoecious types regarding the proportion of pistillate to staminate flowers per plant. In 1954, Janick and Stevenson (1954) reported a study of progeny segregation from selected crosses involving pistillate, staminate, and monoecious types to clarify the genetic mechanisms that bring about the monoecious complex in spinach. Sex determination in dioecious strains of spinach is controlled by a mechanism that acts as if it were a single factor pair; the pistillate plant is homozygous (XX), and the staminate plant is heterozygous (XY). The monoecious character in spinach appears to be controlled by one major gene, Xm, which was found to be allelic to the X Y factor pair. Xm is incompletely dominant to X. The Y allele is dominant to X and Xm.
While breeding efforts to date have provided a number of useful spinach lines with beneficial traits, there remains a great need in the art for new lines with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality.